Origin of Sexual Reproduction

Transcription

1 Origin of Sexual Reproduction Lukas Schärer! Evolutionary Biology Zoological Institute University of Basel Advanced-level Evolutionary Biology What is sexual reproduction? 2 from various websites

2 What is sexual reproduction? 3 the life cycle of the green algae Chlamydomonas Summary what is sexual reproduction?! the historical sequence bacterial mitosis bacterial recombination mitosis syngamy meiosis mating types anisogamy 4

3 The historical sequence sexual reproduction involves many different phenomena the historical sequence of evolutionary events probably was asexual reproduction (binary cell division, mitosis) limited recombination in bacteria fusion between genetically dissimilar cells (syngamy) meiosis (with segregation and recombination) mating types anisogamy, sexual selection, and gender differences when we think about sexual reproduction, we usually think of organisms that have gone through this entire sequence of events but this does not help us to understand where the origin of sexual reproduction lies 5 Bacterial mitosis cell division by bacterial mitosis in prokaryotes 6 from Fogel & Waldor 2006

4 Bacterial recombination plasmids can be transferred from one bacterium to another during bacterial conjugation the ability to fuse with other cells is often a function of the exchanged genetic material itself could bacterial conjugation be of parasitic origin? 7 from Stearns & Hoekstra 2005 Mitosis cell division by mitosis in diploid eukaryotes mitosis 8 from Stearns & Hoekstra 2005

5 Syngamy during syngamy two haploid cells fuse completely brings together genetic material of two (usually) different individuals eventually requires a process that allows this information to be separated again in a coordinated and fair way 9 from Stearns & Hoekstra 2005 Meiosis a direct comparison between meiosis and mitosis meiosis mitosis?? 10 from Stearns & Hoekstra 2005

6 Did syngamy or meiosis evolve first? endomitosis and one-step meiosis occurs in some single-celled eukaryotes e.g. dinoflagellates and sporozoans advantageous in a variable environment diploidy may protect against DNA damage (or rather, it may facilitate DNA repair) haploidy may allow faster growth initially, recombination (crossing-over) probably offers no advantage unknown if it occurs in extant systems 11 from Maynard Smith & Szathmáry 1995 DNA repair and recombination gene conversion is involved in DNA repair and it uses cellular mechanisms that are similar to recombination so recombination could have originated from DNA repair mechanisms 12 from Devoret in Michod & Levin 1988

7 endomitosis one-step meiosis >1 endomitoses followed by >1 meioses 13 The tree of eukaryotes from Keeling et al Did syngamy or meiosis evolve first? endomitosis and one-step meiosis occurs in some single-celled eukaryotes e.g. dinoflagellates and sporozoa advantageous in a variable environment diploidy may protect against DNA damage (or rather, it may facilitate DNA repair) haploidy may allow faster growth initially, recombination (crossing-over) probably offers no advantage unknown if it occurs in extant systems syngamy evolves due to an advantage of heterosis (hybrid vigour) or possibly selfish genetic elements now, recombination may offer an advantage 14 from Maynard Smith & Szathmáry 1995

8 Selfish genetic elements and the evolution of sex in an asexual species a selfish genetic element can only spread within the genome of its host so it would benefit from inducing its host to fuse with another cell and to thereby spread to the other cell s uninfected genome this is true whether the gene has a positive, neutral of negative effect on the host the resulting diploidy may actually be advantageous for the fused cells, because it allows DNA repair and can lead to heterosis but the selfish genetic element only benefits if the cells divide again the division may also allow the host to get rid of mutations via segregation and recombination selfish genetic elements may have been important at the origin of sexual reproduction, but they are probably not responsible for its current maintenance 15 following Hickey & Rose in Michod & Levin 1988 Meiosis segregation and recombination segregation increases with chromosome number chromosome number varies dramatically n=1 in nematode (Parascaris) n=127 in hermit crab (Eupagurus) taxa with uniform chromosome numbers n=13 in most dragonflies n=18 in most snakes taxa with variable chromosome numbers 2n=7 in males and 2n=6 in females of Muntiacus muntjac vaginalis 2n=46 in Muntiacus reevesi segregation 16 recombination from Stearns & Hoekstra 2005

9 Did syngamy or meiosis evolve first? endomitosis and one-step meiosis occurs in some single-celled eukaryotes e.g. dinoflagellates and sporozoa advantageous in a variable environment diploidy may protect against DNA damage (or rather, it may facilitate DNA repair) haploidy may allow faster growth initially, recombination (crossing-over) probably offers no advantage unknown if it occurs in extant systems syngamy evolves due to an advantage of heterosis (hybrid vigour) or possibly selfish genetic elements now, recombination may offer an advantage two-step meiosis evolves may limit the spread of sister-killer alleles 17 Maynard Smith & Szathmáry 1995 Sister-killer alleles 2-step meiosis 1-step meiosis or reductional division (no recombination) equational division (with recombination) kill here kill here 18 following Haig & Grafen 1991

10 Sister-killer alleles 2-step meiosis 1-step meiosis or reductional division (no recombination) equational division (with recombination) kill here or kill here 19 following Haig & Grafen 1991 So much has already happened and still no real sex mitosis two-step meiosis segregation and recombination syngamy 20

11 Mating types initially gametes were probably isogamous and could fuse with all other conspecific gametes (including self gametes) but if the aim is to exchange genes, then selfing is not helpful this may have selected for an incompatibility system avoid fusing with yourself having many mating types would allow fusing with gametes from many partners but surprisingly, we often find that there are only two mating types (usually called plus and minus) this means that every second cell one encounters is a unsuitable fusion partner 21 modified from Mating types genomic conflict with cytoplasmic genetic elements, such as mitochondria and chloroplasts, may explain why there are often only two mating types a possible solution is uniparental inheritance of these cytoplasmic genetic elements usually only from one mating type in Chlamydomonas the mitochondria are inherited by the and chloroplasts are inherited by the mating type 22

12 Mating types a simple route to evolving two mating types 23 from Maynard Smith and Szathmary 1995 Mating types hypotrich ciliates have two kinds of sexual processes conjugation with multiple mating types, but without the exchange of any cytoplasm complete fusion of cells with only two mating types 24

13 Anisogamy: the origin of the male and female sex sexual reproduction involves the fusion between two haploid germ cells to form a diploid zygote sex can occur via isogamy or anisogamy from the Greek iso (=equal), aniso (=unequal) and gamos (=marriage) most organisms that we are familiar with are anisogamous but both types exist even today within, for example, the green algae isogamy is very likely the ancestral condition isogamous and anisogamous gametes in the green algae 25 modified from Anisogamy: the origin of the male and female sex the male sex makes the many small germ cells and the female sex makes fewer large germ cells so the male and female sexes actually only exist in anisogamous species sexual reproduction is possible without the sexes eggs and sperm of Drosophila bifurca bird eggs and sperm 26 from Bjork & Pitnick 2006

14 Anisogamy: the origin of the male and female sex evolution of anisogamy disruptive selection can lead to the evolution of a cell type that specialises in swimming (small) and another that specialises in providing energy (large) selection against fusion between two small cells (low viability) fusion between large cells rare (and therefore also disadvantageous) the small cells (or the individuals making them) compete for access to the large cells (or the individuals making them) sexual selection 27 after Parker 1982 Anisogamy: the origin of the male and female sex 28 from Lessels et al. 2009

15 Anisogamy: the origin of the male and female sex the male sex can be seen as a parasite of the investment by the female sex the evolution of anisogamy involves a primordial sexual conflict (Parker 1979) the evolution of anisogamy also represents the origin of the twofold cost of sex the main cost of sexual reproduction only appeared after evolution of anisogamy, so sex initially may not have needed a big advantage 29 from Summary what is sexual reproduction?! the historical sequence bacterial mitosis bacterial recombination mitosis syngamy meiosis mating types anisogamy 30

16 Literature Mandatory Reading Pages in Chapter 9 on The origin of sex and the nature of species in Maynard Smith & Szathmáry (1995). The Major Transitions in Evolution. Freeman. Suggested Reading Pages of Chapter 8 on The evolution of sex in Stearns & Hoekstra (2005). Evolution: An Introduction. 2nd Edition. Oxford University Press Chapter 10 by Hickey & Rose on The role of gene transfer in the evolution of eukaryotic sex in Michod & Levin (1988). The Evolution of Sex. Sinauer. Books Michod & Levin (1988). The Evolution of Sex. Sinauer. 31